An image edge (contour) detection technique is known widely as a feature amount detection means employed in image recognition and so on. For edge detection, a scheme which treats as an edge a pixel where the luminance gradient of an image takes an extremum is generally known. A smoothing process such as Sobel filter is often conducted as a pre-process in order to eliminate the influence of noise.
A moving picture photographed by a digital camera or the like is saved after it is encoded for the purpose of volume compression. Examples of an encoding means include a scheme called Moving Picture Expert Group (MPEG-2) employed for a Digital Versatile Disk (DVD)-VIDEO and an H.264 scheme which is a scheme employed for terrestrial digital broadcasting (one-segment broadcasting) aimed at mobile terminals and a Blu-ray (registered trademark) Disk. These encoding means adopt a compression method based on motion compensation that utilizes similarities between picture frames. An encoded image signal includes motion vectors indicating the positional relationship between similar portions of the frames, and differential values.
In detecting an edge from an encoded moving picture, an encoded image signal need be decoded, and a similar process need be conducted. For example, Patent Literature 1 discloses a method of detecting an edge of an image by calculating a gradient of the luminance signal of an actual image.